The present invention generally relates to semiconductor materials and, more specifically, to photovoltaic (PV) cells and optoelectronic devices grown on high-miscut-angle substrates.
The interest in PV cells or solar cells has been increasing due to concerns regarding pollution and limited available resources. This interest has been in both terrestrial and non-terrestrial applications. In space applications, the use of nuclear or battery power greatly increases a spacecraft's payload for a given amount of required power to operate the satellite. Increasing the payload of a spacecraft in this manner increases the cost of a launch more than linearly. With the ready availability of solar energy in outer space for a spacecraft such as a satellite, the conversion of solar energy into electrical energy may be an obvious alternative to an increased payload.
The cost per watt of electrical power generation capacity of photovoltaic systems may be a main factor, which inhibits their widespread use in terrestrial applications. Conversion efficiency of sunlight to electricity can be critically important for terrestrial PV systems, since increased efficiency usually results in a reduction of related electricity generation system components (such as cell area, module or collector area, support structures, and land area) for a given required power output of the system. For example, in concentrator photovoltaic systems which concentrate sunlight from around 2 to around 2000 times onto the PV cell, an increase in efficiency typically results in a proportionate reduction of an area comprising expensive concentrating optics.
To increase the electrical power output of such cells, multiple subcells or layers having different energy band gaps have been stacked so that each subcell or layer can absorb a different part of the wide energy distribution in the sunlight. This situation is advantageous, since each photon absorbed in a subcell corresponds to one unit of charge that is collected at the subcell operating voltage, which is approximately linearly dependent on the band gap of the semiconductor material of the subcell. Since the output power is the product of voltage and current, an ideally efficient solar cell would have a large number of subcells, each absorbing only photons of energy negligibly greater than its band gap.
The most efficient and therefore dominant multijunction (MJ) PV cell technology is the GaInP/Ga(In)As/Ge cell structure. Here the use of parentheses in the Ga(In)As middle subcell material indicates the incorporation of indium in the middle cell is optional, so that the composition of the middle cell may be either GaAs or GaInAs. These monolithic cells may be grown lattice-matched to GaAs or Ge, and may have only the top two junctions active with an inactive Ge substrate (2-junction cells), or all three junctions may be active (3-junction cells). While variations on this material system, such as AlGaInP or lattice-mismatched GaInP top cells, might provide a more ideal match of band gaps to the solar spectrum, practical considerations have indicated that lattice-matched GaInP is preferred for large-scale production. Addition of even small amounts of aluminum to the top cell to form AlInGaP simultaneously incorporates oxygen and thus quickly degrades the minority-carrier lifetime and performance of the device. Lattice-mismatched GaInP top cells induce dislocation formation having a similar effect.
In monolithic, series-interconnected, 2- and 3-junction GaInP/Ga(In)As/Ge solar cells, it is desirable for the GaInP top subcell to have nearly the same photogenerated current density as the Ga(In)As subcell. If the currents are different, the subcell with the lowest photogenerated current will limit the current through all of the series-interconnected subcells in the multifunction (MJ) cell, and excess photogenerated current in other subcells is wasted. Limiting the current in this manner results in a severe penalty on the MJ cell efficiency.
At the lattice constant of Ge (or of GaAs) substrates, GaInP grown under conventional conditions has an ordered group-III sublattice and therefore has a band gap which is too low to achieve the desired current match between subcells in the unconcentrated or concentrated AM0 space solar spectrum, the unconcentrated or concentrated AM1.5D and AM1.5G terrestrial solar spectra, and other solar spectra, unless the top subcell is purposely made optically thin, as in U.S. Pat. No. 5,223,043. To achieve the highest efficiencies, the thickness of the subcells in MJ cells are tuned to match the current in each subcell. It is preferable to current match the subcells by increasing the band gap of the top cell rather than reducing its thickness, producing a higher voltage at the same current. An important property of GaInP is that its band gap varies with growth conditions. GaInP grown under conventional conditions is GaInP with a CuPtB ordered group-III sublattice. The result of this ordering may be a decrease in band gap of up to 470 meV for completely ordered material compared with completely disordered material. A. Zunger, MRS Bulletin, 22, (1997) p. 20-26. Typically, this loss in band gap is only 120 meV since the ordering is only partial. The amount of ordering contained in a sample is described by the order parameter, η, which ranges from 0 (disordered) to 1 (completely ordered). G. B. Stringfellow, MRS Bulletin, 22, (1997) p. 27-32.
If the GaInP top cell is fully disordered, an optically thick top cell is nearly current matched for the AM1.5D and AM1.5G terrestrial spectra, but still must be slightly optically thin to match the AM0 spectrum. The increase ΔEg in band gap results in an increase in open-circuit voltage Voc of approximately ΔEg/q (typically 100 mV) for fully-disordered GaInP as compared to partially-ordered GaInP.
Whether in the multiple-junction or single-junction PV device, a conventional characteristic of PV cells has been the use of a window layer on an emitter layer disposed on the base of the PV cell. The primary function of the window layer is to reduce minority-carrier recombination (i.e., to passivate) the front surface of the emitter. Additionally, the optical properties of the window material must be such that as much light as possible is transmitted to lower cell layers where the photogenerated charge carriers can be collected more efficiently, or if there is substantial light absorption in the window, the minority-carrier lifetime in the window must be sufficiently long for the carriers to be collected efficiently at the p-n junction between the emitter and base of the PV cell. Similarly, a back-surface field (BSF) structure below the PV cell base has been used to reduce minority-carrier recombination at the back surface of the base. As for the window, the BSF structure (referred to here simply as a BSF, for brevity) must have optical properties which allow most of the light that can be used by the subcells beneath the BSF to be transmitted by the BSF, and/or the minority-carrier properties in the BSF must be such that electrons and holes which are generated by light absorption in the BSF are efficiently collected at the p-n junction of the PV cell.
For the multiple-cell PV device, efficiency may be limited by the requirement of low resistance interfaces between the individual cells to enable the generated current to flow from one cell to the next. Accordingly, in a monolithic structure, tunnel junctions have been used to minimize the blockage of current flow. In addition to providing the lowest resistance path possible between adjacent subcells, the tunnel junction should also be transparent to wavelengths of light that can be used by lower subcells in the MJ stack, because of the poor collection efficiency of carriers photogenerated in the tunnel junction region.
These properties are all dependent on the band gap, doping levels, optical properties, and minority-carrier recombination and diffusion properties of the base, emitter, window, BSF, and tunnel junction layers employed in the device. The semiconductor properties of these cell layers may be enhanced or degraded for a MJ PV device by the choice of substrate orientation.
Lightly-doped GaInP grown on the conventionally-used Ge substrates, i.e., with a surface orientation that is intentionally tilted by about 6° from the (100) plane toward one of the {111} planes, is quite highly ordered under the growth conditions typically used to produce it. Typically, the observed order parameter is about 0.5 for this case. Conventional methods to increase the output and efficiency of PV cells by disordering the GaInP top subcell of the PV cell include high zinc (Zn) doping and diffusion. Such Zn doping and diffusion, however, alters the material properties of the GaInP top subcell (and potentially other subcells and layers) resulting in non-ideal output and efficiency of the PV cell. A limitation of such a conventional method includes incomplete disordering of the GaInP group-III sublattice, resulting in a lower bandgap and cell voltage than possible with a more complete disordering. In addition, the top subcell device parameters and manufacturability of the MJ cell can be negatively impacted by the requirement to have a high Zn concentration in all or part of the base of the GaInP top subcell.
Another conventional method to increase the output and efficiency of PV cells includes disordering a GaInP top subcell of the PV cell by increasing the GaInP growth rate and growth temperature. In such situations, the disordering of the GaInP top subcell remains incomplete unless rather extreme growth conditions are used, thus placing constraints on the MJ cell growth process that are adverse to the cell's output and efficiency. For example, high growth temperature can degrade the performance of other subcells in the MJ stack, and high growth rates can impose inconveniently high levels of group-III source flows during growth.
The general effect of substrate orientation on sublattice disorder in GaInP has been previously described in the following references: “Sublattice Ordering in GaInP and AlGaInP: Effects of Substrate Orientation,” by Suzuki, et al. and “Competing Kinetic and Thermodynamic Processes in the Growth and Ordering of Ga0.5In0.5P,” by Kurtz, et al. Neither of these references, however, describe MJ photovoltaic cells with higher efficiency grown on high-miscut-angle substrates, than on conventionally-miscut substrates.
To successfully fabricate a MJ cell with a wide-bandgap GaInP top subcell due to substrate misorientation, the effect of misorientation on other materials growth and device properties should be understood, such as effects on nucleation, doping concentration, incorporation efficiency of gallium (Ga) and indium (In), etc.,
U.S. Pat. No. 4,915,744 describes single-junction GaAs cells on substrates that have a special orientation toward a crystallographic plane midway between the (110) and (111) crystallographic planes. No mention is made, however, of GaInP subcells and the effect of substrate misorientation on group-III sublattice ordering of GaInP.
As can be seen, there exists a need for improved single-junction and multifunction photovoltaic cells and other semiconductor devices grown on substrates having surfaces with a crystal orientation significantly misoriented with respect to the (100) plane (herein referred to as high-miscut-angle substrates or high-miscut substrates) in order to alter the materials properties of the grown semiconductor layers, and thereby improve the output and efficiency of PV cells or other type of semiconductor devices.